Carbonyl-forming Eliminations

Hydroxy-THISs add regioselectively to the C=N bonds of isocyanates or isothiocyanates. The initially formed cycloadducts eliminate carbonyl sulfide with formation of 4-hydroxy- or 4-mercaptoimidazolium hydroxide inner salts (21) (Scheme 21). 4-Hydroxyimidazolium hydroxide... 

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

Carbanions derived from phosphine oxides also add to carbonyl compounds. The adducts are stable but undergo elimination to form alkene on heating with a base such as sodium hydride. This reaction is known as the Horner-Wittig reaction.268... 

The Julia olefination involves the addition of a sulfonyl-stabilized carbanion to a carbonyl compound, followed by elimination to form an alkene.277 In the initial versions of the reaction, the elimination was done under reductive conditions. More recently, a modified version that avoids this step was developed. The former version is sometimes referred to as the Julia-Lythgoe olefination, whereas the latter is called the Julia-Kocienski olefination. In the reductive variant, the adduct is usually acylated and then treated with a reducing agent, such as sodium amalgam or samarium diiodide.278... 

Fig. 9.1. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrates. Pathway a Solvolytic reaction (Reaction a) with formation of a carbonium ion, which subsequently undergoes SN1 addition of a nucleophile (e.g., HO ) (Reaction b) or proton E1 elimination to form an olefin (Reaction c). Pathway b HO -catalyzed hydrolysis (,SN2). Pathway c The bimolecular carbonyl-elimination reaction, as catalyzed by a strong base (e.g., HO or RO ), which forms a carbonyl derivative and nitrite.

In oxidation studies it has usually been assumed that thermal decomposition of alkyl hydroperoxides leads to the formation of alcohols. However, carbonyl-forming eliminations of hydroperoxides, usually under the influence of base, are well known. Of more interest, nucleophlic rearrangements, generally acid-catalyzed, have been shown to produce a mixture of carbonyl and alcohol products by fission of the molecule (6). For l-butene-3-hydroperoxide it might have been expected that a rearrangement (Reaction 1) similar to that which occurs with cumene hydroperoxide could produce two molecules of acetaldehyde. 

The transformations of o-iodoalkenylbenzenes in the presence of CO and palladium catalysts may involve either carbonylative cylization, or intramolecular Heck reaction, as well as a number of intramolecular pathways leading to oligomeric byproducts. Non-carbonylative pathways can be reasonably suppressed by applying elevated pressures of GO. Under such conditions, the products are formed in good yields, and the predominant termination stage is Pd hydride / -elimination to form exo-cycWc double bond (Equation (27)). 

The 2-O-methylaldoses undergo //-elimination to form the 2-methyl ethers of the enol form of the 3-deoxyhexosuloses.163 These ethers are relatively stable benzilic acid rearrangement is not possible, because a carbonyl group cannot form at C-2, and further dehydration by elimination of the 4-hydroxyl group, which occurs readily in acid, does not proceed in base.69 Although formation of these ethers has been demonstrated in many studies,164-171 their reactions after... 

The crystal structures of thiamin-dependent enzymes (see next section) as well as modeling102 103 suggest that lactylthiamin pyrophosphate has the conformation shown in Eq. 14-21. If so, it would be formed by the addition of the ylid to the carbonyl of pyruvate in accord with stereoelectronic principles, and the carboxylate group would also be in the correct orientation for elimination to form the enamine in Eq. 14-21, step b.82 83a A transient 380- to 440-nm absorption band arising during the action of pyruvate decarboxylase has been attributed to the enamine. 

The levels of 1,5-asymmetric induction in the palladium-catalyzed alkoxy-carbonylations of alkenols to form 2,6-disubstituted tetrahydropyrans have been shown to be quite reasonable (Table 10 and equation 50).144 Recent studies have shown that cyclization with palladium(II) acetate in DMSO in the absence of CO results in controlled -hydride elimination to form vinyl-substituted tetrahydropyrans with high levels of 1,4- and 1,5-asymmetric induction (equation 51).144b... 

Far greater (Z)-selectivity is observed in reactions of the anion of lb, in which the (Z)/(E) ratio is 20 1. Moreover, the selectivity of this reaction is almost completely reversed by addition of HMPT. The effect of HMPT is exerted on the carbonyl addition step and not on the subsequent elimination to form the double bond. Corey and Rucker1 suggest that lithiated lb reacts mainly as a lithio allene in THF, but mainly as the propargylic anion in THF-HMPT. 

Secondary amines react with ketones that contain an H atom in the a-position through an addition and subsequent El elimination to form enamines (Figure 9.29). In order for enamines to be formed at all in the way indicated, one must add an acid catalyst. In order for them to be formed completely, the released water must be removed (e.g. azeotropically). The method of choice for preparing enamines is therefore to heat a solution of the carbonyl compound, the amine, and a catalytic amount of toluenesulfonic acid in cyclohexane to reflux in an apparatus connected to a Dean-Stark trap. Did someone say Le Chateher ... 

Unlike many other type of radical addition reactions, the product is most often an alkyl-cobalt(III) species capable of further manipulation. These product Co—C bonds have been converted in good yields to carbon-oxygen (alcohol, acetate), carbon-nitrogen (oxime, amine), carbon-halogen, carbon-sulfur (sulfide, sulfinic acid) and carbon-selenium bonds (equations 179 and 180)354. Exceptions to this rule are the intermolecular additions to electron-deficient olefins, in which the putative organocobalt(III) species eliminates to form an a,/ -unsaturated carbonyl compound or styrene353 or is reduced (under electrochemical conditions) to the alkane (equation 181)355. 

Mechanistically, the formation of the double carbonylation products results from CO insertion into the arylmetal species, followed by amine or alkoxide attack on a coordinated CO ligand to form 46, which then undergoes reductive elimination to form the observed products (Scheme 6)366 370 371. 

Previous reactions in this chapter have involved only addition of the nucleophile and a hydrogen to the carbonyl group. In this reaction, addition is followed by elimination of the oxygen to form a double bond between the carbonyl carbon and the nucleophile. Such an addition-elimination reaction occurs when the nucleophile has or can generate (by the loss of a proton or a phosphorus group) a second pair of electrons that can be used to form a second bond to the electrophilic carbon. In the case of the Wittig reaction, the phosphorus and the oxygen are eliminated to form the alkene. The forma-... 

As illustrated using arrow pushing, the first methyl anion drives an addition-elimination reaction forming a ketone. The second methyl anion then adds to the carbonyl in a 1,2-addition, generating the final alcohol. 

The situation is further complicated because some of the initial nucleophilic addition adducts are unstable and undergo elimination to form a stable product. For example, amines (RNH2) add to carbonyl groups in the presence of mild acid to form unstable carbinolamines, which readily lose water to form imines. This addition-elimination sequence replaces aC=0 byaC=N. The details of this process are discussed in Section 21.11. 

The currently accepted mechanism of the Wittig reaction involves two steps. Like other nucleophiles, the Wittig reagent attacks an electrophilic carbonyl carbon, but then the initial addition adduct undergoes elimination to form an alkene. Mechanism 21.4 is drawn using Ph3P=CH2. 

Another variation of the Wittig reaction is the Wittig-Horner reaction, in which the anion generated ot- to phosphine oxide is used as a nucleophile to react with carbonyl compounds. The intermediate formed in this reaction, -hydroxyphosphine oxide, is isolable particularly when bases with lithium counterion are used for deprotonation. Since the j6-hydroxyphosphine oxides are diastereomers, they can be separated and subjected to elimination to form the corresponding alkenes. Since the elimination of phosphonate moiety is syn, stereospecific alkenes are obtained from the elimination step. As expected, the generation of erythro and threo isomers is dependent on the solvent and the reaction conditions. 

Lactones are normally stable compounds, which have found ample application as synthetic intermediates, and, quite recently, have been detected as the central structural unit in physiologically active natural products like obaflorin (123) and lipstatin (124). Characteristic applications of 3-lactones in synthesis are the stereospecific CO2 elimination to form di- and tri-substituted alkenes (e.g. from 125 equation 40) or Grignard addition to the carbonyl group e.g. equation 41). Particularly useful is the formation of 3-lactone enolates (126), which react with a variety of electrophiles (EX) wiA high stereocontrol (equation 42). Organocuprates may be used in chain elongations to form 3-branched carboxylic acids (equation 43). ... 

In this case, a 3-hydroxyl carbonyl compound has been formed. This product has another a-hydrogen, which may be subsequently removed by the base. Then one of two things may happen either this new anion may react with another molecule of ethanal and so form a trimer or a hydroxide anion may be eliminated to form an a,P-unsaturated carbonyl compound. The detailed mechanism for the elimination step is covered in the next chapter so, for the present purposes, simply indicate the product that would result from the elimination of the hydroxide anion. 

The principal methods for forming the carbon-tin bond involve the reaction of organo-metallic reagents with tin compounds (equation 4-1), the reaction of stannylmetallic compounds with organic halides (equation 4-2), the reaction of tin or tin(II) compounds with alkyl halides (equation 4-3), the hydrostannation of alkenes or alkynes (equation 4-4), the reaction of acidic hydrocarbons with Sn-0 and Sn-N bonded compounds (equation 4-5), and carbonyl-forming eliminations (equation 4-6) the symbol sn represents 4Sn. 

Analogous carbonyl-forming eliminations occur with the adducts formed between organotin oxides or alkoxides and trihalogenomethyl aldehydes or ketones, and tributyl-trichloromethyltin was first prepared by the elimination of methyl trichloroacetate from the adduct of tributyltin methoxide and hexachloroacetone.135 136... 

Peroxyesters derived from primary or secondary alkyl hydroperoxides undergo a carbonyl-forming elimination under basic conditions, but hydroysis can be achieved... 

Figure 4.28 Top view of the carbonyl-forming elimination surface.

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